Chlorine is the most widely accepted sanitizers and also one of the most effective in use. Chlorine was discovered in 1774 by Carl Sheele, a German scientist, by mixing muriatic acid and powdered pyrolusite. Though at the time he didn't know what he had discovered, the yellowish green gas would later be called chlorine. Chlorine would not be used as a sanitizer in water until the early 1900'S. Chlorine in modern days has been chemically engineered for maximum sanitation and oxidation. After reading the explanations below, you will have a better grasp of what's happening in the water.

Water Balance of Chlorine

pH: 7.2 - 7.6
Calcium Hardess:100-300
Alkalininty: 80 - 120
Ideal Chlorine Level: 1.0-3.0ppm

The Basic Reaction

As stated earlier, chlorine exists as a gas in its purest form. When this gas enters water it undergoes a simple reaction: water + chlorine = hypochlorous acid + hydrochloric acid. This reaction is the most important reaction of chlorine and has kept pools clean for years. The active ingredient that we want to pay attention to here is the hypochlorous acid (HOCl). It goes through a second reaction: hypochlorous acid <=> hydrogen ion + hypochlorite ion (OCl). This is a reversible reaction that must stay in equilibrium, so as HOCl is used up, more OCl will be converted back to to HOCl, and vice-versa. The most potent, sanitizing form of chlorine, or HOCl, is what you want in your water for maximum disinfection. In fact, depending on the condition, HOCl can be anywhere from 20-200 times more effective at killing microorganisms than OCl. The water balance of your pool has a lot to do with how much killing power (HOCl) you have in your pool.

The Effects of pH on Chlorine One of the most common problems with the use of chlorine is the effects of the pH on the killing power of chlorine. The pH of your water is critical when it comes to chlorine and other sanitizing systems. As I stated earlier, the reaction of chlorine must stay in an equilibrium. That equilibrium is influenced by the pH and can make or break your pool. The lower the pH, the more HOCl you have in your water. However, that does not mean you should go out right now and throw a bunch of acid your pool. Too low of a pH will cause much more serious problems than algae! The correct range for pH (7.2 to 7.6) still applies, but what you probably didn't know is that you can gain 20% more HOCl by keeping your pH around the lower part of the range. If you do the math you're gaining a tremendous amount of killing power that will suppress bacteria and kill algae.

There are other things to think about when keeping the pH lowered. Have you ever heard of chloramines? If not, I'll discuss it later in more detail, but for now just know that they are the nasty form of chlorine that inhibits HOCl from working. The main thing that you need to keep in mind when maintaining the lower pH is that chloramines appreciate it. So while your getting the necessary killing power, you might also be forming a chemical that eats away at your chlorine level. Tricky, huh? The best thing to do is just understand these things, so in the future you know what is happening in your pool. Now you know how to get the maximum killing power, but to also be careful about chloramines.

Types of Chlorine

Gas

There are several different forms of chlorine, but there are just three ways chlorine is commonly sold. Chlorine gas, or Cl2, was first developed and is 100% pure chlorine. Though dispensing chlorine in its gas form was popular in chlorine's earlier days, it is not used much in modern day pools as the equipment that dispenses the gas is quite expensive. The equipment, a lot of times, is difficult to maintain, and justifying the cost to keep a gas system running is even more difficult for most, especially when there are more practical ways for chlorine addition.

Liquid

Liquid chlorine, or NaOCl, has become very popular for commercial uses or experienced chlorine users. It can be bought from 10-12% available chlorine. The problem with liquid chlorine is that special care must be taken when storing the it, and its shelf life is not long at all. A lot of pool stores will not sell it for that reason alone. Liquid chlorine must be vented at all times, for it constantly releases gas and will pop the lid off if you're not careful. There is one final problem with liquid chlorine: it can foam. The caustic soda that it creates when it is introduced into the water will react with the lotions that come off your body and can cause foaming.

Tablets

The tablet form of chlorine can come in many ways, and is the method the average pool owner today uses. The most common form is trichloro-s-triazinetrione, trichlor for short,  is a non-stabilized form of chlorine. Other companies used the stabilized version of trichlor also known as trichloroisocyanuric acid. Some chlorine tablets are composed of calcium hypochlorite. Calcium hypochlorite tablets are not recommended because they can cause problems with chlorine feeders, and they can, in some cases, cause cloudy water. Chlorine tablets are great for dispersing chlorine in a slower, steady manner. They come in two sizes 1" and 3" tablets. Which is right for me, you ask? Well, 3" tablets are generally created for use with in-line chlorinators. In-line chlorinators have a high flow rate and 3" tablets are made to withstand a high rate of water flowing over their surface at high pressures. Most 3" tabs do not readily dissolve in the average floater. So what goes in a chlorine floater? You guessed it 1" tabs. They have more surface area and dissolve easier than the 3" tabs. If you have a floater, and are using 3" tabs, and have a consistently low chlorine level, you might think about switching to 1" tabs. It is possible to break the tablets you already have into smaller pieces, but be careful and wear rubber gloves (chlorine is corrosive, just in case you haven't heard).

Types of Shock  

Ok, so you know what type of chlorine to use, but now your local pool professional is telling you to use shock. What is shock? Well simply it is a way to raise your chlorine level, oxidize and sanitize your water, and is a great way to kill and/or prevent algae. When referring to different types of shock pay attention to the active ingredient and not what brand it is. Though brands can make a difference, you need to know the type of shock your using. As you already know with anything what you pay for is what you get. The same principal applies to shock. The rest of this page will help you chose what type of shock to use with your pool.

Calcium Hypochlorite

The cheapest, most common, and also the most problematic shock is calcium hypochlorite.  Its chemical formula is Ca(OCl)2. When cal-hypo is introduced into water it creates hypochloric acid, one calcium molecule, and a hydroxyl ion. So let's start out positive with the good things about this shock. Calcium hypochlorite, or cal-hypo for short, is a stable shock that has a great shelf life of 2-3 years. Cal-hypo has a very high available chlorine percentage, anywhere between 65-75%, which means it is very strong. In fact, it is classified as an "extreme oxidizer," because it is so powerful. The biggest problem with this shock is that calcium does not readily dissipate and is not very soluble. For vinyl pools, not to mention some others, you have to dissolve it before insertion in a 5 gallon bucket. It shortens the length of liners and can bleach painted or unpainted plaster pools. The calcium also can raise the water's hardness level and cause scale. Though, cal-hypo is a stable shock it is not to be mixed with any other chemicals. It can and will cause fires and in extreme cases explosion will occur.

Sodium Dichlor

A great alternative for the cal-hypo shock is sodium dichlor.  Sodium dichloro-s-triazinetrione, or sodium dichlor, is a very stable shock, still relatively cheap (on average $2.75-$4.00 per pound depending on the quantity purchased), and it can be found at almost any pool store. It is most efficient (not to mention, safe) when purchased at 56% available chlorine or less. It does not cloud water, or dissipate slowly. In fact some brands claim that you can swim as soon as one hour after adding. It has a very long shelf life of up to 3 years. One of the best features of this product is that it will not influence the pH significantly. It depends on the brand for whether you should pre-mix it in a 5 gallon bucket, but better to be safe than sorry. For the price, sodium dichlor is one of the most satifactory shocks available.

Lithium Hypochlorite

One of the fastest dissolving shocks on the market is Lithium Hypochlorite. This shock is one of the best, and its price says it loud at about $4.95 a pound. It provides 35% available chlorine and though that doesn't sound like much it packs a strong punch. It is recommended especially for vinyl lined pools because it is highly soluble and will not bleach the liner. In most cases lithium shock actually lengthens a liner's life because it does not deteriorate it like other shocks may do. There is no need to ever pre-mix this shock in a bucket because of how soluble it is.